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DNA Replication

What is DNA Replication?

DNA replication is the process where your cells create exact copies of their genetic code. To do this, your cells rely on the nucleotides that make up your DNA. Nucleotides are composed of a phosphate group, a five-carbon sugar, and one of four nitrogenous bases: adenine, thymine, cytosine, and guanine—often shortened to A, T, C, and G. These bases have a specific binding pattern, where A binds to T, and C binds to G, to form nucleotide pairs. The bound pairs of nucleotides form a twisted ladder-like structure called a double helix, where each rung in the ladder is a pair of bonded nucleotides.

To replicate DNA, the bonds between the nucleotide bases are broken, so that the double helix splits in two. With the help of an enzyme called DNA polymerase, each base is then bonded to a new partner—as before, A with T, and C with G—to form a new strand of DNA. This specific bonding pattern results in two copies of the DNA molecule being created from the original version.

On occasion, DNA polymerase will accidentally match the wrong bases together, such as bonding an A with a C instead of a T. This is a problem, as these kinds of errors can result in a mutation, a heritable change in the DNA. To avoid this, DNA polymerase “proofreads” and stops replicating if it detects an error. Once other enzymes fix the mistake, DNA polymerase can resume and continue until the DNA replication process is complete. This proofreading activity is so effective that, on average, only one mutation occurs for every 100 million bases.

Why Does DNA Replicate?

DNA replication plays an important role in the growth and renewal of cells. Growing organisms are constantly creating new cells as they develop into a larger body. And over time, some cells can become damaged, grow old, or die. To keep your body functioning properly, it’s important that these cells are quickly replaced with new ones. Cells accomplish this renewal and growth through the process of cell division, in which one cell splits in half to form two new cells. In order for a cell to divide, it must first make a copy of its own DNA, which is the genetic code it needs to function properly. It is very important that your DNA is replicated accurately, with new cells receiving an exact copy of your genetic sequence.

How Was the DNA Replication Process Discovered?

For many years, scientists were unsure how a cell replicated its DNA. Three competing theories were proposed. The first theory, known as the conservative model, posited that the original DNA double helix remained completely intact, and the new copy did not borrow any molecules from the original. The semiconservative model suggested that DNA unwound during replication, with each strand serving as a template for new synthesis. Finally, in the dispersive model, the original double helix was believed to be broken into many small pieces, with some of these pieces ending up within each new strand.

The debate was finally resolved in 1958 by two scientists named Matthew Methelson and Franklin Stahl. In a now-famous biology experiment, they grew bacteria inside a special solution to label all of the cells’ DNA with a marker. They then used a different marker to label only the DNA that was newly-synthesized. They discovered that replicated DNA always contained one strand from the original DNA molecule and one strand that was newly formed. This proved that the semiconservative model of DNA replication was correct.

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